Direction sensing method and apparatus for laser doppler velocity measurements

ABSTRACT

Two beat frequencies due to doppler measurement of the velocity component of a moving object are produced and the difference in their frequencies compared to determine the direction of object movement along the path of the velocity component. One beat frequency is provided by producing first and second radiant energy energy fields whose frequency difference is a function of the sign of the velocity component. The other beat frequency is provided by periodically shifting the phase of one of the radiant energy fields variably with respect to time. The radiant energy fields are produced by a laser device and the phase shift is effected by applying a sawtooth charging voltage to a Pockels cell interposed in the path of one of the laser beams.

United States Patent Lehmann [451 Mar. 14, 1972 Bernhard Lehmann,Berlin, Germany Licentia Patentverwaltungs GrnbH, Frankfurt, GermanyFiled: Aug. 18, 1969 Appl. No.: 850,935

Inventor:

Assignee:

[30] Foreign Application Priority Data Aug. 17, 1968 Germany ..P 17 98076.6

[56] References Cited UNITED STATES PATENTS 10/1967 De Maria ..356/281/1968 Badewitz ....343/17.5 4/1969 Eden ..350/ 160 2/1969 Jacobs et a1......250/218 5/1969 Seaton ..356/28 3,419,330 12/1968 Schneider..356/l06 2,886,717 5/1959 Williamson et a1. ..250/237 G FOREIGN PATENTSORAPPLICATIONS 1,953,630 3/ 1970 Germany ..356/28 OTHER PUBLICATIONSRowe et al., Surface Topography Of Non-optical Surfaces By ProjectedInterference Fringes, Nature, Vol. 216, Nov. 25, 1967, pp. 786- 7Primary Examiner-Rodney D. Bennett, Jr. Assistant Examiner-S. C.Buczinski Att0rneySpencer & Kaye [57] ABSTRACT Two beat frequencies dueto doppler measurement of the velocity component of a moving object areproduced and the difference in their frequencies compared to detenninethe direction of object movement along the path of the velocitycomponent. One beat frequency is provided by producing first and secondradiant energy energy fields whose frequency difference is a function ofthe sign of the velocity component. The other beat frequency is providedby periodically shifting the phase of one of the radiant energy fieldsvariably with respect to time. The radiant energy fields are produced bya laser device and the phase shift is effected by applying a sawtoothcharging voltage to a Pockels cell interposed in the path of one of thelaser beams.

14 Claims, 5 Drawing Figures SAM/TOOTH /6ENERA ro/r lNTE/VS/TY SENSITIVERECE/VER Patented March 14, 1972 3,649,125

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BEAM SPL/TTER/ SAM/TOOTH T GENERATOR INTENSITY SENS/T/VE RECEIVE/iInventor.-

Bunhanl Lehmann B y= fl mmm {7407s fliimneys Patented March 14, 1972 2Sheets-Sheet 2 DIRECTION SENSING METHOD AND APPARATUS FOR LASER DOPPLERVELOCITY MEASUREMENTS BACKGROUND OF THE INVENTION Doppler velocitymeasuring processes employing radiant energy are well known and dopplerlaser processes employing optical radiation are in particular normallylimited to the determination only of the magnitude of the dopplerfrequency change. That is to say, since the velocity of moving objectsnormally encountered is very small compared with the frequency of theoptical radiation, the doppler frequency is in the order of thefrequency of light and, as a result, the time resolution capability ofoptical-electronic receiving instruments is insufficient to determinethe positive or negative aspect of the velocity component of the movingobject. Thus, although the magnitude of the doppler frequency and itsmeasurement allow the determination of the path of the velocitycomponent, it does not provide any information concerning which of thetwo directions the object is moving along the path, i.e., whether thevelocity is positive or negative.

SUMMARY OF THE INVENTION The present invention is directed to method andapparatus for overcoming the above deficiency, Essentially, the presentinvention concerns the production of two sequential beat frequencysignals; each of which may be of such high frequency that the aboveproblem would be encountered with either, but whose difference infrequency, or frequency spectrum, is small and may be determined andwhich will provide the necessary information asto the positive ornegative aspect of the velocity component being measured. The first beatfrequency may be produced by known methods which involve the detectionof doppler frequency difference between two radiant energy fieldswherein the doppler frequency is related to the velocity component ofthe moving object. The second beat frequency is produced by phaseshifting one of the radiant energy fields variably with respect to time.Dependent upon the nature of the phase shift, comparison between thefrequencies of the two beat frequencies will establish the positive ornegative aspect of the velocity component.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic viewillustrating the prior art.

FIG. 2 is a view similar to FIG. 1 but illustrating the principles ofthe present invention.

FIG. 3 is an illustration of the sawtooth waveform applied to thePockels waveform of FIG. 2.

FIGS. 40 and 4b are views illustrating the visual displays of frequencyspectrum analyzing devices which may be used to determine the positiveor negative aspect of the velocity component being measured.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates a prior artdevice in which two-directional radiation impingement on the movingobject 4 is employed. The laser beam 1, of frequency f is divided intothe two partial beams 3 and 3 by means of the beam divider 2, suchpartial beams being directed toward the moving object 4 under theconvergence angle a. The object 4 is moving at velocity V in thedirection shown along a path perpendicular to the plane which is at theangle B with respect to the angle bisector ofthe angle a. Due todispersion upon impingement on the object 4, the two partial beams 3 and3 produce two spatial, scattered light fields 5 and 5 whose frequenciesare doppler-shifted to different degrees due to the different directionsof impingement The intensity-sensitive receiver 6 measures the intensityof the beat frequency Af produced by interference between the twoscattered light fields 5 and 5, the intensity being equal to thedoppler-shifted frequency.

If n is the index of refraction of the medium surrounding the object andA is the wavelength of the laser light in vacuum: Af=2 Vn/B (cos B sin01/2). (I)

With the arrangement of FIG. 1, it will be appreciated that the exactposition and path of the velocity component may be determined fromequation (1). However, to determine the positive or negative aspect ofthe velocity component to establish which direction the object is movingalong its path, the sign of Af would have to be known. That is, if f isthe frequency of the field 5 and f that of the field 5', with Af=f f thesign of Af, whether positive or negative, is necessary to complete theinformation as to the velocity component with the arrangement of FIG. 1.However, contemporary opticalelectronic means are incapable ofdetermining the sign of Af and the measured value is always [A f I.

This drawback becomes particularly troublesome when the velocity vectorof an object is being measured in space by means of three-componentmeasurement by radiation upon the object from three directions. When thenegative or positive aspects of the individual measurements are unknown,there is an eight-fold inaccuracy possibility.

The above problem is present with other optical arrangements which maybe employed. For example, a common arrangement involves the replacementof one of the scattered light fields of FIG. 1 by a reference beam whichis uninfluenced in frequency by the moving object but which is derivedby splitting the original laser beam and incorporation thereof in thereceived scattered light.

With reference to FIG. 2, the present invention may take the form inwhich a phase-shifting device 7 is interposed in the path of the partiallaser beam 3. The device 7 may be continuously or intermittentlycontrolled by the device 8 so that at successive times, a first beatfrequency is produced by the fields 5 and 5 as described above and thena second beat frequency is produced by phase-shifting of one of thefields in the fashion hereinafter described. As will also be shownhereinafter, the difference in frequencies between these two beatfrequencies may be compared readily and their comparison will providethe necessary data as to the sign of Af in equation l The two partialbeams in FIG. 2 provide the two scattered light fields 5 and 5 bydispersion, the electrical field intensities of these fields oscillatingapproximately according to:

2 2 2 2) where t time, A amplitude, to angular frequency (w 21rf), andd) phase shift. The frequency difference Aw= w, m is measured by theintensity-sensitive receiver 6.

The intensity which results from the superposition of both fields is I(E E and when all members with frequencies in the order of magnitude ofthe laser light frequency which can not be resolved are neglected:

The phase difference Ad: is arbitrary in the laser doppler measurementsand is constant in time for the moment of measurement so that it is notincorporated in the frequency and is of no interest. However, byconstraining the phase difference of the second beat frequency to betime-dependent, that is by making Ada A(t) with known sign, the sign ofA0) 2-rrf may be established and thus the incapacity of the prior artarrangements of determining the sign of Af by frequency measurement isovercome.

In the simplest case, the phase shift which produces the second beatfrequency may be linearly variable with time, i.e., A(t) I 1+ C, where Iand C are constants. The constant C may be disregarded so that equation(5) becomes, for the second beat frequency:

I cos (Aw t, where Aw b is the new, or second beat frequency:

A0. Aw I 7 Since knowledge of the sign of D is presupposed, thefollowing relations may be established:

nan-s These relations are unequivocal as long as I D I 2 Aw which may berealized readily within the state of the present art.

To achieve the above, the device 7 of FIG. 2 may be an elecltro-opticalcell, for example a Pockels cell, which is interposed in the path of oneof the partial beams 3 or 3' or in the reference beam of the other priorart embodiment described above. The Pockels cell 7 is excited by ahigh-frequency sweep generator having a voltage output waveform of thetype shown lll FIG. 3. The index of refraction of the cell 7 varieslinearly with the positive-going ramp 10 of the sawtooth waveform which,as shown, increases linearly with time so that during the time intervalof the ramp 10, the second beat frequency M), as above described, isproduced.

The two beat frequencies Am and A0. may be observed. for example, in aspectrum analyzer such as an oscillograph, examples being illustrated inFIGS. 40 and 4b. Assuming for either figure that 1 0 as established bythe waveform of FIG. 3, and noting that ;A (ll A 11 relationship (8)above indicates that Aw 0 which establishes the required remaininginformation in connection with the velocity component, l.e., Aw 21rf ispositive. This information plus the information provided in the usualfashion as per equation l completely identifies the velocity componentbeing measured. The choice of phase shift which is linear with respectto time is advantageous in that the observed frequency change produces afixed frequency image with constant Am. However. it is to be understoodthat phase shifting which is non-linear with respect to time may beemployed ifdesired.

With the arrangement as described. relative frequency changes A Di A mlA ml of percent to percent may be obtained easily for values of Amnormally encountered by proper selection of the dimensions of the cell7. and the maximum voltage and frequency of the sawtooth waveform.

The negative-going ramp 11 of the sawtooth waveform 18 of such shorttime duration that a frequency lS produced. This frequency may causeambiguities and, in such case, the laser light beam may be cut offduring this time. Thus, the first beat frequency AQ would be producedduring a portion of the time of the positive-going ramp l0 and duringwhich time such ramp voltage is not applied to the cell 7; the secondbeat frequency Aw would be produced during a subsequent portion of theramp time; by short-time keying of the sawtooth voltage, i.e., byapplying the sawtooth voltage to the lPockels cell 7 for a short periodof time, and, as noted, the laser light partial beam through the cell 7would be cut offdurmg the time ofthe ramp [1.

Alternatively, the sawtooth waveform may be designed such llS to producethe beat frequency due to the ramp 11, in which case the beat frequencyA!) may be distinguished easily without requiring the laser light to becut offduring ramp 11 time.

it will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations.

lclaim:

l. A method for determining the positive or negative aspect of thevelocity component of a moving object, comprising the steps of:

producing first and second radiant energy fields whose interferenceprovides a beat frequency which is a function of the velocity componentofa moving object; periodically shifting the phase of one of saidradiant energy Fields as a variable with respect to time to provide asecond beat frequency which also is a function of the elocity componentof the moving ob'ect; and comparing the first and second beat requenciesto deter mine the positive or negative aspect of the velocity componentof the moving object.

2. The method according to claim 1 wherein the phase of said one radiantenergy field is shifted in phase linearly with respect to time.

3. The method according to claim 2 wherein said first and second radiantenergy fields are effected by laser beams, the phase shift beingeffected by a Pockels cell charged with a sawtooth voltage waveform.

4. The method as defined in claim 3, wherein the phase of said one ofsaid radiant energy fields is periodically shifted by short time keyingof the sawtooth voltage applied to the Pockels cell influence on thefrequency.

5. The method as defined in claim 1 wherein said one of said radiantenergy fields is periodically shifted in phase prior to the impingementof same on said moving object.

6. The method according to claim 1 wherein said first and second radiantenergy fields are effected by laser beams.

7. Apparatus for determining the positive or negative aspect of thevelocity component of a moving object, comprising, in combination:

means for producing first and second radiant energy fields whoseinterference produces a first beat frequency which lS a function of thevelocity component of a moving obyect;

means for periodically shifting the phase of one of said radiant energyfields as a function of time to produce a second beat frequencydifferent from said first beat frequency and which is also a function ofthe velocity component of the moving object; and

means for determining, from the frequency spectrum of said first andsecond beat frequencies, the positive or negative aspect of the velocitycomponent being measured. 8. Apparatus according to claim 7 wherein saidone radiant energy field is phase-shifted linearly with respect to time.

9. Apparatus according to claim 8 wherein the first mentioned means is alaser device.

10. Apparatus according to claim 7 wherein the first mentioned means isa laser device.

11. Apparatus according to claim 10 wherein said laser device producesfirst and second beams which produce said first and second radiantenergy fields; and wherein said phase shifting means includes a Pockelscell interposed in the path of one of said beams, and means forperiodically charging said cell with a sawtooth voltage waveform.

12. The apparatus as defined in claim 8 wherein said phase shiftingmeans periodically shifts the phase of said one of said radiant energyfields prior to the impingement of same on said moving object.

13. A doppler laser velocity measurement system for determining thepositive or negative aspect of a velocity component of a moving objectwhose velocity is small compared to the velocity of light, comprising,in combination:

means for producing interfering optical fields providing first andsecond beat frequencies whose frequency difference lS very much lessthan the laser light frequency; and

means for determining the positive or negative aspect of said velocitycomponent from said frequency difference of the beat frequencies. 14. Amethod of doppler laser velocity measurement for determining thepositive or negative aspect of a velocity component ofa moving objectwhose velocity is small compared to the velocity oflight, comprising thesteps of:

producing interfering optical fields providing first and second beatfrequencies whose frequency difference is very much less than the laserlight frequency; and

determining the positive or negative aspect of said velocity componentfrom said frequency difference of the beat frequencies.

(not:

1. A method for determining the positive or negative aspect of thevelocity component of a moving object, comprising the steps of:producing first and second radiant energy fields whose interferenceprovides a beat frequency which is a function of the velocity componentof a moving object; periodically shifting the phase of one of saidradiant energy fields as a variable with respect to time to provide asecond beat frequency which also is a function of the velocity componentof the moving object; and comparing the first and second beatfrequencies to determine the positive or negative aspect of the velocitycomponent of the moving object.
 2. The method according to claim 1wherein the phase of said one radiant energy field is shifted in phaselinearly with respect to time.
 3. The method according to claim 2wherein said first and second radiant energy fields are effected bylaser beams, the phase shift being effected by a Pockels cell chargedwith a sawtooth voltage waveform.
 4. The method as defined in claim 3,wherein the phase of said one of said radiant energy fields isperiodically shifted by short time keying of the sawtooth voltageapplied to the Pockels cell influence on the frequency.
 5. The method asdefined in claim 1 wherein said one of said radiant energy fields isperiodically shifted in phase prior to the impingement of same on saidmoving object.
 6. The method according to claim 1 wherein said first andsecond radiant energy fields are effected by laser beams.
 7. Apparatusfor determining the positive or negative aspect of the velocitycomponent of a moving object, comprising, in combination: means forproducing first and second radiant energy fields whose interferenceproduces a first beat frequency which is a function of the velocitycomponent of a moving object; means for periodically shifting the phaseof one oF said radiant energy fields as a function of time to produce asecond beat frequency different from said first beat frequency and whichis also a function of the velocity component of the moving object; andmeans for determining, from the frequency spectrum of said first andsecond beat frequencies, the positive or negative aspect of the velocitycomponent being measured.
 8. Apparatus according to claim 7 wherein saidone radiant energy field is phase-shifted linearly with respect to time.9. Apparatus according to claim 8 wherein the first mentioned means is alaser device.
 10. Apparatus according to claim 7 wherein the firstmentioned means is a laser device.
 11. Apparatus according to claim 10wherein said laser device produces first and second beams which producesaid first and second radiant energy fields; and wherein said phaseshifting means includes a Pockels cell interposed in the path of one ofsaid beams, and means for periodically charging said cell with asawtooth voltage waveform.
 12. The apparatus as defined in claim 8wherein said phase shifting means periodically shifts the phase of saidone of said radiant energy fields prior to the impingement of same onsaid moving object.
 13. A doppler laser velocity measurement system fordetermining the positive or negative aspect of a velocity component of amoving object whose velocity is small compared to the velocity of light,comprising, in combination: means for producing interfering opticalfields providing first and second beat frequencies whose frequencydifference is very much less than the laser light frequency; and meansfor determining the positive or negative aspect of said velocitycomponent from said frequency difference of the beat frequencies.
 14. Amethod of doppler laser velocity measurement for determining thepositive or negative aspect of a velocity component of a moving objectwhose velocity is small compared to the velocity of light, comprisingthe steps of: producing interfering optical fields providing first andsecond beat frequencies whose frequency difference is very much lessthan the laser light frequency; and determining the positive or negativeaspect of said velocity component from said frequency difference of thebeat frequencies.